The earth is composed of various kinds of elements. These elements occur in solid form in the outer layer of the earth and in hot and molten form in the interior. About 98% of the earth’s crust is composed of eight major elements — oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium. The remaining portion consists of elements such as titanium, hydrogen, phosphorous, manganese, sulphur, carbon and nickel.The elements present in the earth’s crust are generally found in combined form and produce various substances known as minerals. A mineral is a naturally occurring organic or inorganic substance having a definite chemical composition, orderly atomic structure and distinct physical properties. Most minerals are composed of two or more elements, although some single-element minerals such as sulphur, copper, silver, gold and graphite are also found.Although the lithosphere is made up of a limited number of elements, these combine in different ways to form nearly 2,000 identified minerals. Most commonly occurring minerals belong to the major rock-forming mineral groups.The basic source of all minerals is the hot magma present in the earth’s interior. As magma cools, mineral crystals form in a systematic sequence and eventually solidify into rocks. Some minerals such as coal, petroleum and natural gas are organic substances found in solid, liquid and gaseous forms respectively.
The external shape of minerals is determined by the internal arrangement of molecules. Minerals may occur as cubes, octahedrons, hexagonal prisms and other forms.
Cleavage refers to the tendency of minerals to break along definite planes, producing relatively smooth surfaces. This results from the internal molecular arrangement. Cleavage may occur in one or more directions and at different angles.
When the internal arrangement of molecules is highly complex and no cleavage planes exist, the mineral breaks irregularly. This property is known as fracture.
Lustre refers to the appearance of a mineral surface irrespective of colour. Minerals may show metallic, silky, glossy and other types of lustre.
Some minerals possess a characteristic colour because of their molecular structure, such as malachite, azurite and chalcopyrite. In some minerals, colour results from impurities. For example, quartz may appear white, green, red or yellow.
Streak refers to the colour of the powdered form of a mineral. It may or may not match the actual colour of the mineral. For example, malachite gives a green streak, while fluorite, though purple or green, produces a white streak.
Minerals may be:
Structure refers to the arrangement of mineral crystals. Minerals may be fine-grained, medium-grained, coarse-grained, fibrous, divergent or radiating.
Hardness indicates the resistance of a mineral to scratching. Ten minerals are used on a scale from 1–10:
A fingernail has hardness 2.5, while glass or a knife blade has hardness 5.5.
Specific gravity is the ratio between the weight of a mineral and the weight of an equal volume of water.
These minerals contain metal content and are divided into three categories:
Include gold, silver and platinum.
Include iron and metals mixed with iron to produce different forms of steel.
Include copper, lead, zinc, tin and aluminium.
These minerals do not contain metallic content. Examples include sulphur, phosphates and nitrates. Cement is a mixture of non-metallic minerals.
The earth’s crust is composed of rocks, which are aggregates of one or more minerals. Rocks may be hard or soft and occur in different colours. For example:
Unlike minerals, rocks do not possess a definite chemical composition. Feldspar and quartz are among the most common minerals found in rocks.The study of rocks is known as Petrology, and a scientist who studies rocks is called a Petrologist. Petrology deals with the mineral composition, texture, structure, origin, occurrence, alteration and relationships of rocks.Because of the close relationship between rocks and landforms as well as rocks and soils, knowledge of rocks is important in geography.
Rocks are grouped into three major families on the basis of their mode of formation:
Igneous rocks are known as primary rocks because all other rocks originate from them. The term Ignis means fire in Latin. These rocks form when magma cools and solidifies either within the earth’s crust or on the earth’s surface.The texture of igneous rocks depends upon the size and arrangement of mineral grains, which in turn depends upon the rate of cooling:
Examples of igneous rocks include:
The term sedimentary comes from the Latin word sedimentum, meaning settling. Existing rocks are broken into fragments by denudational agents. These fragments are transported by exogenous agencies and deposited in layers. Over time, these deposits become compacted and transformed into rocks through the process of lithification.Sedimentary rocks generally preserve their layered structure even after lithification. Therefore, rocks such as sandstone and shale often display distinct layers of varying thickness.
Examples:
Examples:
Examples:
The term metamorphic means change of form. These rocks are formed when pre-existing rocks undergo changes due to pressure, volume and temperature (PVT) conditions.Metamorphism occurs when:
Metamorphism leads to recrystallisation and reorganisation of minerals within the rocks.
Mechanical breaking and rearrangement of minerals occurs without major chemical change.
Chemical alteration and recrystallisation occur due to heat.
Occurs when rocks come in contact with hot magma or lava.
Occurs due to tectonic shearing, high temperature and pressure over large regions.
Arrangement of minerals in layers or lines.
Alternating layers of minerals producing light and dark bands. Rocks showing such structure are called banded rocks.Metamorphic rocks are broadly divided into:
Examples include:
The rock cycle is a continuous process through which old rocks are transformed into new rocks.
Thus, rocks continuously undergo transformation through melting, cooling, weathering, deposition, compaction and metamorphism.